Marker structure for optical alignment of a substrate, a substrate including such a marker structure, an alignment method for aligning to such a marker structure, and a lithographic projection apparatus

ABSTRACT

A marker structure on a substrate for optical alignment of the substrate includes a plurality of first structural elements and a plurality of second structural elements. In use, the marker structure allows the optical alignment based upon providing at least one light beam directed on the marker structure, detecting light received from the marker structure at a sensor, and determining alignment information from the detected light, the alignment information comprising information relating a position of the substrate to the sensor.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application No. 60/411,861,filed Sep. 20, 2002, and U.S. Application No. 60/413,601, filed Sep. 26,2002, both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a marker structure for opticalalignment of a substrate, a substrate including such a marker structure,an alignment method for aligning to such a marker structure and alithographic projection apparatus.

2. Description of the Related Art

The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

Another example of a patterning device is a programmable mirror array.One example of such an array is a matrix-addressable surface having aviscoelastic control layer and a reflective surface. The basic principlebehind such an apparatus is that, for example, addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate filter, the undiffracted light can be filtered out of thereflected beam,eaving only the diffracted light behind. In this manner,the beam becomes patterned according to the addressing pattern of thematrix-addressable surface. An alternative embodiment of a programmablemirror array employs a matrix arrangement of tiny mirrors, each of whichcan be individually tilted about an axis by applying a suitablelocalized electric field, or by employing piezoelectric actuators. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors. In this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronics. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be seen, forexample, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and WO 98/38597and WO 98/33096. In the case of a programmable mirror array, the supportstructure may be embodied as a frame or table, for example, which may befixed or movable as required.

Another example of a patterning device is a programmable LCD array. Anexample of such a construction is given in U.S. Pat. No. 5,229,872. Asabove, the support structure in this case may be embodied as a frame ortable, for example, which may be fixed or movable as required.

For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningdevice may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(e.g. comprising one or more dies) on a substrate (silicon wafer) thathas been coated with a layer of radiation-sensitive material (resist).In general, a single wafer will contain a whole network of adjacenttarget portions that are successively irradiated via the projectionsystem, one at a time. In current apparatus, employing patterning by amask on a mask table, a distinction can be made between two differenttypes of machine. In one type of lithographic projection apparatus, eachtarget portion is irradiated by exposing the entire mask pattern ontothe target portion at once. Such an apparatus is commonly referred to asa wafer stepper. In an alternative apparatus, commonly referred to as astep-and-scan apparatus, each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection. Since, in general, the projection system will have amagnification factor M (generally<1), the speed V at which the substratetable is scanned will be a factor M times that at which the mask tableis scanned. More information with regard to lithographic devices as heredescribed can be seen, for example, from U.S. Pat. No. 6,046,792.

In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. It is important to ensure that the overlay juxtaposition) of thevarious stacked layers is as accurate as possible. For this purpose, asmall reference mark is provided at one or more positions on the wafer,thus defining the origin of a coordinate system on the wafer. Usingoptical and electronic devices in combination with the substrate holderpositioning device (referred to hereinafter as “alignment system”), thismark can then be relocated each time a new layer has to be juxtaposed onan existing layer, and can be used as an alignment reference.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

For the sake of simplicity, the projection system may hereinafter bereferred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and6,262,796.

For a lithographic process, an alignment of the wafer to be processedwith the mask pattern on the mask should be as precise as possible for acorrect definition of features on the substrate, which features allshould have sizes within specified tolerances. To this end, thelithographic projection apparatus includes a wafer alignment modulewhich provides for alignment of the substrate with the mask and maskpattern within a given (specified) tolerance. The wafer alignment systemtypically performs the alignment based on optical devices. The positionof a wafer or a portion of a wafer is determined by measuring an opticalresponse from an optical marker which is illuminated by an opticalsource: for example, a grating is illuminated by a laser beam, the laserbeam diffracts from the grating, and one or more of the diffractedorders are measured by respective sensors, which are typically locatedon a reference plane. Using the output of the sensors the position ofthe wafer can be derived (relative to the reference plane).

In the prior art, optical markers include a grating with a periodicitysuitable for diffraction of impinging light with a wavelength wellwithin the visible range of the spectrum. A typical periodicity is 16μm. The grating is typically constructed of lines and trenches.Typically, the line width and trench width are each 8 μm. In order toobtain sufficient diffracted light from the grating and to obtainwell-defined diffraction maxima and minima, the grating must encompass aminimal number of lines and intermediate trenches. The size in thedirection of the periodic structure is about 750 μm.

The grating may be a phase grating or phase marker which takes intoaccount a phase difference between the phase of rays scattered at theupper surface of the grating and the phase of rays scattered at thelower surface of the grating.

Also, the grating may be an amplitude grating which only takes intoaccount the periodic structure of the grating without any further phasedifference relating to an upper or lower level in the grating.Typically, an amplitude grating or amplitude marker is constructed of aperiodic structure of first and second elements, which have similarsurface levels but different respective reflectance.

Optical markers are used during microelectronic device processing (or ICprocessing) along the full manufacturing line. During the front end ofline (FEOL), markers are used for alignment during manufacturing oftransistor structures. At a later stage during the back end of line(BEOL), markers are needed for alignment of metallisation structures,e.g. connect lines, and vias. It is noted that in both cases theintegrity of the markers must be sufficient to meet the requiredaccuracy of alignment.

During semiconductor manufacturing processes a wafer is subjected to aplurality of treatments such as annealing, etching, polishing, etc.,which may likely cause roughness of a marker (a recessed area in themarker and/or warping of the marker). Such marker roughness causes analignment error of the image which may contribute to an overlay error inthe construction of a semiconductor device. Also, it is conceivable thatduring the subsequent stages of processing the quality of markers tendto diminish.

A disadvantage of prior art optical markers is that during IC processingit is difficult to control the phase depth of the optical marker. As aresult, the intensity of diffracted light under a given diffractionangle may be low, and even close to zero, and accurate measurement ofthe diffracted beam may be difficult, or even impossible. The phasedepth can be defined as the resolved height difference between a topsurface of a line and a top surface of a trench in a grating under agiven angle of diffraction. If under an angle of diffraction, where(under optimal conditions) a maximum of diffracted intensity isexpected, the phase depth equals half a wavelength of the appliedradiation, interference between diffracted waves will result in a low orzero intensity.

Control of the phase depth during IC processing may be difficult due toprocess variations from wafer to wafer, and also across a single wafer.

A further disadvantage of prior art markers results from the dependenceof marker properties as a function of the layer(s) below the marker. Itis known that due to different optical behaviour of the various layers,as found in semiconductor devices, the contrast of the marker may vary,which results in variations of the diffracted intensity as function ofthe layer below.

Moreover, it is known that various processing steps may adverselyinfluence the shape of alignment markers. Due to the effect on theshape, the alignment by such modified markers may comprise an errorwhich can result from the fact that the modified shape of the markerchanges the generated (pattern of) diffracted beams.

Furthermore, in the prior art, during BEOL processing, optical markerscould be detected under capping layers by virtue of the residualstructure which was visible at the surface. However, due to applicationof planarisation processes such as chemical mechanical polishing (CMP),the option to use a residual marker structure for alignment in manycases has become impossible.

In the prior art, markers on a semiconductor substrate that comprisetrenches filled with tungsten, are subjected to a CMP process forremoving tungsten from the surface and planarising the surface. Due tothe CMP process, the tungsten structures are either filled orunderfilled. The extent of filling is related to the phase depth of anoptical signal generated by the marker, i.e., two discrete phase depthlevels exist. One level relates to filled tungsten structures which areshallow and have a small phase depth, the other level relates tounderfilled tungsten structures which are relatively deep and have alarge phase depth. Small phase depth of the filled markers isundesirable since the alignment error caused by the small phase depth isrelatively large. Also, the large phase depth does not guarantee thatthe alignment error is reduced: the phase depth may be such thatextinction of the optical signal results. Furthermore, no control overthe phase depth is obtained.

The influence of optical markers on IC processing may lead toundesirable side effects due to the fact that the optical markers areinherently larger than feature sizes in integrated circuits. In theprior art, the minimum feature size of markers is in the order of 1 μm.In current microelectronic devices, the typical minimal feature size isabout 100 nm (depending on the device generation). Since the markerusually includes the same material as (part of) the devices, thepresence of an additional marker area of a substantial size in thevicinity of a device may have an influence on the local processing ratefor that device in a given processing step. For example, a chemicalreaction in a reactive ion etching process or a chemical depositionprocess may be influenced by the presence of a large marker area due tosome kinetic constraint, or due to a local deviation of the wafertemperature, etc. A size difference between marker and device featuremay thus lead, for example, to modification of a processing step fordevices located closely to a marker. Due to the modification of theprocessing a variation of device characteristics may occur across a dieand/or a wafer.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. One of ordinary skill in the art willappreciate that, in the context of such alternative applications, anyuse of the terms “reticle”, “wafer” or “die” in this text should beconsidered as being replaced by the more general terms “mask”,“substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “projection beam” areused to encompass all types of electromagnetic radiation, includingultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having awavelength in the range 5-20 nm).

SUMMARY OF THE INVENTION

It is an aspect of the present invention to provide a marker structurethat allows correction of the phase depth in such a way that negativeinterference within the diffraction pattern is prevented.

In a first aspect of the present invention, a marker structure on asubstrate for optical alignment of the substrate includes a plurality offirst structural elements and a plurality of second structural elements,in use the marker structure allowing the optical alignment based uponproviding at least one light beam directed on the marker structure,detecting light received from the marker structure at a sensor,determining alignment information from the detected light, the alignmentinformation including information relating a position of the substrateto the sensor, wherein the first structural element has a firstreflecting surface on a first level and a second reflecting surface on asecond level lower than the first level, the second structural elementis substantially non-reflecting, a separation between the first andsecond reflecting surfaces determines a phase depth condition for thedetected light, and a recess is provided in the second reflectingsurface to modify the phase depth condition.

The recessed area changes the phase depth by an amount that produces apositive interference with sufficient intensity seen at the sensor.

In a second aspect of the present invention, a marker structure on asubstrate for optical alignment of the substrate includes a plurality offirst structural elements and a plurality of second structural elements,in use the marker structure allowing the optical alignment based uponproviding at least one light beam directed on the marker structure,detecting light received from the marker structure at a sensor,determining alignment information from the detected light, the alignmentinformation including information relating a position of the substrateto the sensor, wherein the first structural element has a firstreflecting surface on a first level and a second reflecting surface on asecond level lower than the first level, the second structural elementis substantially non-reflecting, a separation between the first andsecond reflecting surfaces determines a phase depth condition for thedetected light, and the second reflecting surface includes a pluralityof additional structural elements located above an opaque layer.

Advantageously, the variation of the detected intensity as a function ofthe underlying layer is reduced by the stacking of marker structuresabove each other. The intermediate dielectric layer can be tuned to havean optimal phase depth with positive interference.

It is a further aspect of the present invention to provide a markerstructure that allows monitoring of processing-induced damage. Accordingto this aspect, a marker structure on a substrate for optical alignmentincludes a plurality of first structural elements and a plurality ofsecond structural elements, in use the marker structure allowing theoptical alignment based upon providing at least one light beam directedon the marker structure, detecting light received from the markerstructure at a sensor, determining alignment information from thedetected light, the alignment information including information relatinga position of the substrate to the sensor, wherein the marker structureincludes a first periodic structure and a second periodic structure, thesecond periodic structure is adjacent and parallel to the first periodicstructure, the first periodic structure includes a plurality of firststructural elements of a first material having a first width and aplurality of second structural elements of a second material having asecond width, the first and second structural elements are arranged in arepetitive order, with the first width being larger than the secondwidth, the second periodic structure includes a plurality of the firststructural elements of the second material having a third width and aplurality of the second structural elements of the first material havinga fourth width, the first and second structural elements being arrangedin a repetitive order, the third width being equal to the first widthand the fourth width being equal to the second width, and the firststructural element in the second periodic structure being locatedadjacent to the first structural element in the first periodic structurein such a manner that the second periodic structure is complementary tothe first periodic structure.

By such a complementary structure, which includes a first periodicstructure and a second periodic structure complementary to the firstone, it is possible to monitor by use of the alignment system if one ofthe structural elements within a periodic structure is damaged by the ICprocessing sequence, since a diffraction pattern will change differentlyfor the first periodic structure than for the second periodic structurewhen damage occurs on either the first or the second structural elementwithin the first and the second periodic structure.

It is another aspect of the present invention to provide a markerstructure that overcomes the removal of a residual marker structure froma metallization layer due to CMP processing of the underlying layerwhich includes the marker structure. According to this aspect, a markerstructure includes a plurality of first structural elements and aplurality of second structural elements, in use the marker structureallowing the optical alignment based upon providing at least one lightbeam directed on the marker structure, detecting light received from themarker structure at a sensor, determining alignment information from thedetected light, the alignment information including information relatinga position of the substrate to the sensor, wherein the marker structureis present in a metallization layer, the first structural elementincludes a first surface area portion having a first surface state andthe second structural element includes a second surface area portionhaving a second surface state, the first surface area portion beingrelated to a first buried marker element, and the second surface areaportion being related to a second buried marker element, the first andthe second surface state being related to variations in morphology ofthe metallization layer being induced by the first buried marker elementand the second buried marker element, respectively.

Advantageously, the metallization layer is deposited in such a way thata difference in surface state/morphology as a function of the underlyingmaterial is created in that metallization layer. The periodic variationof surface state/morphology of the surface is detectable by an alignmentand/or overlay system.

It is an aspect of the present invention to provide a marker structurethat overcomes effects caused by a relatively large marker area onfeatures of devices located in the vicinity of such a large marker area.According to this aspect, a marker structure includes a plurality offirst structural elements and a plurality of second structural elements,in use the marker structure allowing the optical alignment based uponproviding at least one light beam directed on the marker structure,detecting light received from the marker structure at a sensor,determining alignment information from the detected light, the alignmentinformation including information relating a position of the substrateto the sensor, wherein the first structural element comprises aplurality of primary lines and a plurality of first interposed lines.

Advantageously, the structural elements that form the marker structureare each sub-divided in sub-elements which have a characteristic sizecomparable to the product feature size. By mimicking the feature size ofthe product more closely, size-induced processing effects are minimised.

Moreover, it is an aspect of the present invention to provide alithographic projection apparatus which allows application of the markerstructure as described above.

Furthermore, it is an aspect of the present invention to provide amethod of alignment of a substrate in a lithographic projectionapparatus which uses the marker structure as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the following drawings, wherein:

FIG. 1 depicts a lithographic projection apparatus comprising at leastone marker structure according to the present invention;

FIG. 2 schematically shows a cross-sectional view of a prior art markerstructure to illustrate the concept of phase depth;

FIG. 3 a schematically shows a cross-sectional view of a markerstructure from the prior art in copper-damascence layers;

FIG. 3 b schematically shows a cross-sectional view of a markerstructure according to a first embodiment of the present invention:

FIG. 3 c schematically shows a cross-sectional view of a markerstructure according to a second embodiment of the present invention;

FIG. 4 schematically shows a cross-sectional view of a marker structureaccording to a third embodiment of the present invention;

FIG. 5 schematically shows a marker structure according to a fourthembodiment of the present invention in a perspective view;

FIG. 6 schematically shows a planar view of a marker structure accordingto a fifth embodiment of the present invention;

FIG. 7 a schematically shows a cross-sectional view of a filled andunderfilled tungsten marker from the prior art beforechemical-mechanical polishing of tungsten;

FIG. 7 b schematically show a planar view of a tungsten marker structurein silicon dioxide according to a sixth embodiment of the presentinvention;

FIG. 7 c schematically shows a cross-sectional view of the tungstenmarker structure in silicon dioxide of FIG. 7 b;

FIG. 8 shows a planar view of a marker structure according to a seventhembodiment of the present invention;

FIG. 9 shows an application of a stack of markers in accordance with theseventh embodiment of the present invention.

In the Figures, like symbols indicate like parts.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the present invention. The apparatusincludes a base plate BP. The apparatus may also include a radiationsource SO (e.g. UV or EUV radiation, such as, for example, generated byan excimer laser operating at a wavelength of 248 nm, 193 nm or 157 nm,or by a laser-fired plasma source operating at 13.6 nm). A first object(mask) table MT is provided with a mask holder configured to hold a maskMA (e.g. a reticle), and is connected to a first positioning device PMthat accurately positions the mask with respect to a projection systemor lens PL. A second object (substrate) table WT is provided with asubstrate holder configured to hold a substrate W (e.g. a resist-coatedsilicon wafer), and is connected to a second positioning device PW thataccurately positions the substrate with respect to the projection systemor lens PL. The projection system or lens PL (e.g. a quartz and/or CaF₂lens system or a refractive or catadioptric system, a mirror group or anarray of field deflectors) is configured to image an irradiated portionof the mask MA onto a target portion C (e.g., comprising one or moredies) of the substrate W.

The projection system PL is supported on a reference frame RF. As heredepicted, the apparatus is of a transmissive type (i.e. has atransmissive mask). However, in general, it may also be of a reflectivetype, (e.g. with a reflective mask). Alternatively, the apparatus mayemploy another kind of patterning device, such as a programmable mirrorarray of a type as referred to above.

The source SO (e.g. a UV excimer laser, an undulator or wiggler providedaround the path of an electron beam in a storage ring or synchrotron, alaser-produced plasma source, a discharge source or an electron or ionbeam source) produces radiation. The radiation is fed into anillumination system (illuminator) IL, either directly or after havingtraversed a conditioner, such as a beam expander Ex, for example. Theilluminator IL may include an adjusting device AM configured to set theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in the beam. Inaddition, it will generally comprise various other components, such asan integrator IN and a condenser CO. In this way, the projection beam PBimpinging on the mask MA has a desired uniformity and intensitydistribution in its cross-section.

It should be noted with regard to FIG. 1 that the source SO may bewithin the housing of the lithographic projection apparatus (as is oftenthe case when the source LA is a mercury lamp, for example), but that itmay also be remote from the lithographic projection apparatus, theradiation beam which it produces being led into the apparatus (e.g. withthe aid of suitable directing mirrors). The latter scenario is often thecase when the source SO is an excimer laser. The present inventionencompasses both of these scenarios.

In particular, the present invention encompasses embodiments wherein theilluminator IL is configured to supply a projection beam of radiationhaving a wavelength of less than about 170 nm, such as with wavelengthsof 157 nm, 126 nm and 13.6 nm, for example.

The projection beam PB subsequently intercepts the mask MA, which isheld on the mask table MT. Having traversed the mask MA, the projectionbeam PB passes through the lens PL, which focuses the beam PB onto atarget portion C of the substrate W. With the aid of the secondpositioning device PW and interferometric measuring system IF, thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning device PM can be used to accurately position the maskMA with respect to the path of the beam PB, e.g. after mechanicalretrieval of the mask MA from a mask library, or during a scan. Ingeneral, movement of the object tables MT, WT will be realized with theaid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning). However, in the case of a wafer stepper (asopposed to a step and scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed. The mask MA andthe substrate W may be aligned using mask alignment marks M₁, M₂ andsubstrate alignment marks P₁, P₂.

The depicted apparatus can be used in two different modes:

-   -   1. In step mode, the mask table MT is kept essentially        stationary, and an entire mask image is projected at once, i.e.        a single “flash,” onto a target portion C. The substrate table        WT is then shifted in the X and/or Y directions so that a        different target portion C can be irradiated by the beam PB;    -   2. In scan mode, essentially the same scenario applies, except        that a given target portion C is not exposed in a single        “flash.” Instead, the mask table MT is movable in a given        direction (the so-called “scan direction”, e.g. the Y direction)        with a speed v, so that the projection beam PB is caused to scan        over a mask image. Concurrently, the substrate table WT is        simultaneously moved in the same or opposite direction at a        speed V=Mv, in which M is the magnification of the lens PL        (typically, M=¼ or ⅕). In this manner, a relatively large target        portion C can be exposed, without having to compromise on        resolution.

The interferometric measuring system typically can include a lightsource, such as a laser (not shown), and one or more interferometers IFconfigured to determine some information (e.g., position, alignment,etc.) regarding an object to be measured, such as a substrate or astage. In FIG. 1, for example, one interferometer IF is schematicallydepicted. The light source (laser) produces a metrology beam MB which isrouted to the interferometer IF by one or more beam manipulators. Incase more than one interferometer is present, the metrology beam isshared between them, by using optics that split the metrology beam invarious separate beams for each interferometer.

A substrate alignment system MS configured to align a substrate on tableWT with the mask MA on mask table MT, is schematically shown at anexemplary location close to substrate table WT, and includes at leastone light source which generates a light beam aimed at a markerstructure on the substrate and at least one sensor device which detectsan optical signal from that marker structure. It is noted that thelocation of the substrate alignment system MS depends on designconditions which may vary with the actual type of lithographicprojection apparatus. The marker structures may be for example substratealignment marks P1, P2.

FIG. 2 schematically shows a cross-sectional view of a prior art markerstructure to illustrate the concept of phase depth.

An optical marker structure typically includes a grating 50 with aperiodicity P suitable for diffraction of impinging light with awavelength λ. Where the term “light” is used in the present document, itis not limited to wavelengths within the visible spectrum but mayencompass light of longer wavelength or shorter wavelength than visiblewavelengths. It will be appreciated that it is not essential that theperiodicity P be suitable for diffraction of light within the visiblespectrum, and that the invention can be implemented with a periodicity Psuitable for shorter wavelengths or suitable for longer wavelengths.

The grating includes a series of lines 100, with interposed trenches102. The trenches 102 have a depth d_(t) with respect to the top surfaceof the lines 100. The periodicity P of the grating is composed of a linewidth P_(I) and a trench width P_(II).

In FIG. 2, an impinging light beam λ is directed in a directionsubstantially perpendicular to the surface of the substrate.Alternatively, a non-perpendicular inclination of the impinging beam maybe used.

The marker grating from the prior art is a so-called phase grating. Adiffraction pattern is schematically shown by two diffracted beams, eachhaving a diffraction angle θ relative to the surface.

In the diffraction pattern the position of intensity maxima and minimais governed by the periodicity of the grating. When the wavelength λ ofthe impinging light is within the range of visible light, periodicity Pmay typically be 16 μm to obtain a diffraction pattern suitable forpurpose of alignment. Typically, the line width P_(I) and trench widthP_(II) are each 8 μm.

In order to obtain sufficient diffracted light from the grating 50 andto obtain an angular distribution (a diffraction pattern) ofwell-defined diffraction maxima and minima, the grating 50 mustencompass a minimal number of lines 100 and intermediate trenches 102which are illuminated by the impinging light beam. In the prior art, amarker includes at least 10 trenches within the illuminated area.

The intensity of the diffracted beams is further determined by the depthd_(t) of the trenches 102 relative to the top surface of the lines 100.In a certain direction of diffracted light, the light rays scattered atthe top surface of the lines 100 and the light rays scattered at thebottom of the trenches 102 must have a certain phase relation to obtaina positive interference between these light rays in that direction,independent from the periodicity P. The depth d_(t) of the trenches 102relative to the surface of the lines 100 must be such that positiveinterference will occur. If the interference is negative, an extinctionof the signal will occur. This is known as the phase depth condition.

In the phase grating 50, the interference in the diffraction pattern canbe schematically deduced as follows: under angle θ a first set ofphotons reflect on the top surfaces of the lines 100, while a second setof photons reflect at the floor of the trenches 102. In order todetermine if in a given direction, indicated by diffraction angle θ, anintensity maximum or minimum will occur, the phase difference of photonsoriginating from the line top surfaces and from the trench floors mustbe substantially zero or half a wavelength, respectively, at thepropagation front PF.

For an optical marker structure on a semiconductor wafer, the structuremay be exposed to various deformations during the processing steps ofthe semiconductor wafer to form integrated circuits. Due to thesedeformations the phase depth dt may change during manufacturing.

FIG. 3 a schematically shows a cross-sectional view of a markerstructure from the prior art. In FIG. 3 a, an optical marker structure50 on a substrate layer SL is shown which may be applied in back end ofline (BEOL) IC processes for copper (Cu)-based micro-electronic devices.Such devices are typically manufactured by so-called Cu damasceneprocess technology, wherein copper structures are embedded in (opticallytransparent) dielectric material, appearing as a “floating marker”. Theoptical marker 50 includes a plurality of Cu line elements 104, embeddedin dielectric material, wherein the dielectric material is shaped as aplurality of line elements 106. Typically, the dielectric material mayconsist of a stack of various separate dielectric layers. Byconsequence, the phase depth d_(t) of the marker 50 may be ill-defineddue to variations in the thickness of various separate dielectric layersin the dielectric stack. Moreover, variations may exist across a die ora wafer. Thus, in a worst case, the intensity of the marker's signals asincluded in the diffraction pattern may be too weak for detection by thealignment tool. This may result in a marker reject or even a waferreject during IC processing.

FIG. 3 b shows a cross-sectional view of a marker structure according toa first embodiment of the present invention.

A way to avoid extinction due to an incompatible phase depth is shown inFIG. 3 b. In FIG. 3 b entities with the same reference number refer tothe same entities as shown in FIG. 3 a. In the area of the semiconductorsubstrate layer (or in general an opaque layer) SL under the floatingmarker 50, a recess R1 is created in front end of line (FEOL) process.This recess increases the phase depth and thus reduces the probabilityof negative interference between scattered light from the surface leveland the level of the substrate or opaque layer.

As shown in FIG. 3 b, the recess R1 may be formed only under a portionof area covered by the floating marker 50, in which case two differentphase depths are present, one of which may yield a better usablediffraction signal of sufficient intensity.

FIG. 3 c shows a cross-sectional view of a marker structure according toa second embodiment of the present invention.

In the second embodiment, the recess is defined during a FEOL processonly below a portion of the marker 50. As indicated in the right-handside of FIG. 3 c, recesses R2 are formed only below the transparentportions of the marker 50. In the left-hand side of FIG. 3 c, recessesR3 are formed only below the opaque portions of the marker 50. Again,two different phase depths are present, each of which may yield a usablediffraction signal of sufficient intensity.

It is noted that recesses R2, R3 may be formed by using the mask of themarker and an appropriate lithographic process, with a positive ornegative exposure, respectively.

The recesses shown in FIG. 3 b or 3 c preferably add about 200-300 nm tothe phase depth.

A further disadvantage of prior art markers is due to the dependence ofmarker properties as a function of the layer(s) below the marker. It isknown that due to different optical behaviour of the various layers, asfound in semiconductor devices, the contrast of the marker may vary,which results in variations of the diffracted intensity as function ofthe layer below, i.e. the phase depth strongly varies as a function ofthe underlying layer.

FIG. 4 schematically shows a cross-sectional view of a marker structureaccording to a third embodiment of the present invention.

In the third embodiment, the phase depth is better controlled bydefining a first optical marker OM1 in a first metal layer (by exposureand processing) in a first ordering tone, i.e., a given periodicrepetition of a first structural element and a second structuralelement. Next, in a second metal layer stacked on the first one with atleast one intermediate dielectric layer IDL, a second optical marker OM2with the same first ordering tone, but in reverse tone relative to thefirst marker, is defined (exposed and processed). The reverse toneindicates that the second marker OM2 comprises the same periodicrepetition as the first optical marker OM1, but the locations of thefirst structural element and the second structural element are exchangedrelative to the first marker OM1.

By control of the intermediate dielectric layer IDL the phase depth canbe controlled: i.e., a phase depth value is selected which yields adiffraction signal of sufficient intensity. Moreover, the space occupiedby the markers within a scribelane of the wafer is strongly reduced bystacking optical markers.

It is noted that the thickness of the intermediate dielectric layer IDLis usually determined by the IC processing parameters. If, accidentally,the thickness of the intermediate dielectric layer IDL between thestacked markers corresponds to a phase depth which causes negativeinterference for a wavelength used by the substrate alignment system, asecond wavelength may be used.

FIG. 5 schematically shows a marker structure according to a fourthembodiment of the present invention in a perspective view.

Various processing steps during IC fabrication may adversely influencethe shape of alignment markers. For example, the block shape of thelines in the optical marker structure may change due to a CMP step. Dueto the CMP process the cross-section of lines becomes asymmetrical: thepolishing rounds only one of the top edges, basically due to the (local)polishing direction.

Due to the effect on the shape, the alignment by such modified markers(rounded at one edge) may include an error which results from the factthat the modified shape of the marker results in a change of thediffraction pattern being generated. Typically, a modification of theshape of the marker results in a shift of the position of thediffraction peaks generated by the optical marker structure relative tothe position of the peaks for the pristine marker shape. In the priorart, it has not been possible to distinguish between a genuinemisalignment of a marker or a modification of the marker shape, sinceboth events lead to a change of the diffraction pattern and/or peakpositions in the pattern.

The optical marker structure according to the fourth embodiment providesa possibility to check whether the shift of the pattern is due tomisalignment of the marker or due to deformation of the marker inducedby IC processing.

The optical marker includes a first periodic structure PS1 in a firstportion, and a second periodic structure PS2 in a second portion. Thefirst and second periodic structures PS1 and PS2 are located adjacent toeach other with their respective periodicity running parallel in onedirection.

The first periodic structure PS1 has the same periodicity as PS2 butit's ordering of structural elements is complementary to the secondperiodic structure PS2. The first periodic structure PS1 includes aplurality of first structural elements SE1 of a first material with afirst width w1 and a plurality of second structural elements SE2 of asecond material with a second width w2, which are arranged periodically.

The second periodic structure PS2 includes a plurality of thirdstructural elements SE3 of the second material with a third width w3 anda plurality of fourth structural elements SE4 of the first material witha fourth width w4, which are arranged periodically. Since PS1 iscomplementary to PS2, the first structural element SE1 is adjacent tothe third structural element SE3 with the first width w1 being equal tothe third width w3, and the second structural element SE2 is adjacent tothe fourth structural element SE4 with the second width w2 being equalto the fourth width w4. Further, the periodic structures PS1 and PS2 areeach asymmetric: the first and second widths differ from each other.

As an example, the optical marker structure may be arranged as a Cudamascene structure, with Cu as first material, and an insulator as thesecond material; the periodic variation of the Cu and insulator causingthe marker structure to act as a diffraction grating. Thus, for exampleSE1 and SE4 include Cu and SE2 and SE3 e insulator. The width w1 of SE1is equal to the width w3 of SE3, and the width w2 of SE2 is equal to thewidth w4 of SE4.

It is noted, however, that such a marker may also be embodied in ametal-semiconductor structure or a metal-insulator structure. Also, thiscomplementary optical marker structure can be formed by twocomplementary geometric gratings (i.e., lines and trenches) etched inthe semiconductor substrate and located next to each other.

The use of complementary features in the marker structure results in afixed signal (with zero level) during measurement. If the periodicstructures PS1 and PS2 are substantially complementary, a first signalfrom the first periodic structure PS1 will be the complement of a secondsignal from the second periodic structure PS2. The first and secondsignal cancel each other, and the combined signal of first and secondsignal as measured by a sensor will have a substantially zero level.

Due to processing effects on the structure as described above, the firstperiodic structure PS1 changes in a different manner than the secondperiodic structure PS2 because of the different asymmetry of bothstructures. In the first periodic structure PS1 the metal lines SE1 mayhave a different width w1 than the width w4 of the metal lines SE4 inthe second periodic structure PS2. Due to the difference in width of themetal lines and insulator lines in the respective structures PS1 andPS2, the change of the shape of the respective lines will be different.

Due to this different modification of the two structures, the firstsignal from PS1 is no longer the complement of the second signal fromPS2. By consequence, the complementary grating will no longer display azero level signal during measurement. Instead a non-zero signal will bemeasured.

The occurrence of such a signal from the complementary optical markerstructure indicates process-related influences on the marker. Thus thepresence of processing-induced effects on other markers with similarperiodicity and the drift of these effects can advantageously bemonitored by this complementary optical marker structure.

In certain IC metallisation processes, a buried marker (i.e., an opticalmarker structure below the metallisation layer) is still detectable dueto a residual topography at the surface. In that case, the geometricshape of the marker structure i.e., the lines and trenches, are stillvisible in the surface of the metallisation layer as elevated andlow-lying regions, respectively.

However, in IC processing, chemical -mechanical polishing (CMP) isapplied as planarisation technique for W contacts and vias. By CMP thetop surface is flattened, and any residual topography is lost. It hasbecome impossible to align using a residual marker structure in such acase.

FIG. 6 shows a planar view of a marker structure according to a fifthembodiment of the present invention.

In the fifth embodiment of the present invention, a marker structure isformed in the aluminium metallisation layer which acts as an amplitudemarker structure. FIG. 6 shows a layer stack which is formed during BEOLprocessing: in the trenches a tungsten W contact is formed. By CMP thesurfaces of the W contact and insulator Oxide are planarized. On theplanarized surface a Ti adhesion layer is deposited. Next, A1 isdeposited by a hot metal deposition process. Finally, a capping layer ofTi/TiN is deposited. In FIG. 6, some exemplary values for the thicknessof the respective layers are indicated.

The metallization layer includes a hot metal process (deposited byphysical vapor deposition, typically at about 350° C. under UHVconditions). Due to the different grain growth of A1 on the Ti adhesionlayer covering W and covering silicon dioxide, respectively, a differentsurface state is created in the A1 layer depending on the under-lyingmaterial. Above the W contacts or plugs, the surface has a first surfacestate ST2, and above the oxide, the surface has a second surface stateST1.

Possibly, the Ti layer may have a different texture depending on theunderlying material. This texture may influence the nucleation and graingrowth of A1 deposited during a hot metal deposition process in a mannerdifferently for the area covering W and the area covering silicondioxide. The difference in surface state relates to a difference inmorphology, i.e., texture and/or grain size of the metallization layerdepending on the underlying material. Alternatively, the differentnucleation and grain growth of A1 on W or A1 on silicon dioxide may alsobe caused by differences in thermo-physical properties of the underlyingmaterial since the Ti layer is relatively thin.

In either case, due to whatever physical-chemical cause, the localdifference in surface state is detectable by the alignment—and/oroverlay-sensor system as a marker structure.

It is noted that such a morphological marker structure is not limited tothe specific structure described in FIG. 6. The metallization layer mayshow a periodic variation of a surface state also due to some otherunderlying materials (processed by CMP) which form a periodic structure.

In the prior art, markers on a semiconductor substrate that comprisetrenches filled with tungsten, are subjected to a CMP process forremoving tungsten and planarizing the surface of the substrate. Due tothe combination of W-CVD and CMP, the tungsten structures are eitherfilled or underfilled. The extent of filling is related to the phasedepth of an optical signal generated by the marker, i.e., two discretephase depth levels exist.

One level relates to filled tungsten structures which are shallow andhave a small phase depth due to the substantially complete fill up tothe top of the structure.

The other level relates to underfilled tungsten structures which arerelatively deep and have a large phase depth.

Small phase depth of the filled markers is undesirable since thealignment error caused by the small phase depth is relatively large.Also, the large phase depth does not guarantee that the alignment erroris reduced: the phase depth may be such that extinction of the opticalsignal results.

FIG. 7 a shows a cross-sectional view of filled and under filledtungsten markers from the prior art, before CMP of the tungsten W.

In a trench etched within a silicon dioxide layer, tungsten is depositedby a CVD process in blanket mode. FIG. 7 a illustrates that the width ofthe trench governs whether the conformally grown tungsten layer fillsthe trench in a ‘filled’ or ‘underfilled’ mode.

During CVD of tungsten W with conformal growth characteristics, narrowtrenches will become ‘filled’ trenches, while wide trenches will become‘underfilled’.

The bottom of the trench may be covered by a barrier layer.

Next, a CMP process is carried out to planarize the structure. In thismanner, a metal (W) structure with a surface substantially on level withthe silicon dioxide surface is formed. As a consequence, the phase depthof the ‘filled’ structure is substantially zero. The ‘underfilled’ metalstructure comprises portions (i.e., the side-walls) which aresubstantially on level with the silicon dioxide surface, and a centralportion which surface is well below the silicon dioxide surface level.After CMP, the central tungsten W portion has a given phase depthrelative to the silicon dioxide surface level.

For a given depth of the trenches, and for a tungsten deposition processwith given processing parameters (i.e., a conformal W layer with a giventhickness is formed), the width of the trench determines whether atungsten line will either be filled or underfilled. Thus, the phasedepth will comprise two discrete levels as a function of the trenchwidth. Furthermore, due to different resistance to CMP of tungsten andsilicon dioxide, the CMP process can not be controlled very accurately.

As mentioned above, in a marker structure comprising underfilled metalmarker lines the depth of the central portion of the metal line may besuch that the phase depth is substantially zero: no control over thephase depth is obtained.

FIGS. 7 b and 7 c show planar and cross-sectional views, respectively,of a tungsten marker structure in silicon dioxide according to a sixthembodiment of the present invention.

In the sixth embodiment, the optical marker structure includes tungstensub-segments in the silicon dioxide lines.

As sub-segments, a plurality of sub-trenches are formed in the silicondioxide lines, with the length direction of sub-trenches extending in adirection parallel to the periodicity P of the marker structure. Sincethe plurality of sub-trenches are periodically arranged in a directionwhich will be a so-called non-scanning direction during alignmentprocedures, the optical effect of the periodic arrangement of thesub-trenches in that direction Psub is not detected by the substratealignment system. The possible diffraction signal generated by (theperiodicity of) the sub-trenches is directed in a directionperpendicular to the direction of the diffraction signal of the actualmarker structure (i.e., the repetition of tungsten trenches and silicondioxide lines), so this possible signal is not detected by the substratealignment system.

The trenches and sub-trenches are filled with tungsten by a tungsten CVDprocess. Next, a CMP process is carried out to planarize the structure.Due to the presence of tungsten in the sub-trenches, the CMP process isin better control. By using sub-trenches the area of the markerstructure comprising filled tungsten structures with their specificresistance to CMP is relatively enlarged (tungsten is more resistant toCMP than oxide). This allows polishing of the filled trenches to a givenheight with greater accuracy. By better controlling the polished heightof the filled trenches relative to the level of the lower portions ofthe underfilled structures, the phase depth may be controlled. Theheight of the top level of the filled tungsten structure relative to thelower level of tungsten in the underfilled region may be adapted toobtain a desired phase depth. The phase depth may be adapted by changingthe spacing between the sub-trenches (and their number) in the silicondioxide lines to change the relative area of filled W structures.

The width of the sub-trenches is double the thickness of the conformaltungsten layer (which thus results in a completely filled sub-trenchhaving a zero phase depth).

The influence of optical markers on IC processing may lead toundesirable side effects due to the fact that the optical markers areinherently larger than feature sizes in integrated circuits. In theprior art, the minimum feature size of markers is in the order of 1 μm.In current microelectronic devices, the typical minimal feature size isabout 100 nm (depending on the device generation). Since the markerusually includes the same material as (part of) the devices, thepresence of an additional marker area of a substantial size in thevicinity of a device may have an influence on the local processing ratefor that device in a given processing step. For example, a chemicalreaction in a reactive ion etching process or a chemical depositionprocess may be influenced by the presence of a large marker area due tosome kinetic constraint, or due to a local deviation of the wafertemperature, etc. A chemical mechanical polishing process may beinfluenced by the presence of a large marker area due to some mechanicalconstraint (i.e., a higher or lower resistance to CMP) caused by themarker area.

A size difference between marker and device feature may thus lead tomodification of a processing step for devices located closely to amarker. Due to the modification of the processing a variation of devicecharacteristics may occur across a die and/or a wafer.

From the viewpoint of IC manufacturing, a change of dimensions of themarker's structural elements to have them correspond more closely tocritical feature sizes in devices may overcome the problem of the sizedependency of IC processes. However, a change of “line” and “trench”widths may also change the periodicity of the marker. This wouldadversely require a major effort to redesign alignment—andoverlay-sensor systems to comply with a new marker periodicity.

Moreover, since alignment systems use linearly polarized laser light,polarization effects resulting from interaction with such a modifiedmarker structure may adversely result in strongly reduced signalstrengths in such alignment systems.

To overcome this size dependency of IC processes, the inventorsrecognized that the optical marker structure from the prior art isrequired to be segmented in such a way that critical device features arebetter mimicked while the diffraction pattern generated by the modifiedmarker structure remains substantially the same as for an unmodifiedmarker from the prior art. Also, the alignment system is arranged insuch a way that polarization effects results in usable signal strengthof measured signals.

FIG. 8 shows a planar view of a marker structure according to a seventhembodiment of the present invention.

In the seventh embodiment, the first structural elements aresub-segmented in a plurality of primary lines L1 extending in a firstdirection D1, each primary line having a width comparable to criticalfeature sizes of a device. In between the primary lines are interposedlines of a different material. The width of the primary and interposedlines is such that dense device structures with critical feature sizesare mimicked.

Further, the second structural elements in between the first structuralelements are sub-segmented in a plurality of secondary lines L2extending in a second direction D2, with interposed lines of a differentmaterial in between. Again, the secondary lines and interposed lineshave widths comparable to dense device structures with the criticalfeature sizes for devices. The second direction D2 is perpendicular tothe first direction D1.

The material of the primary lines L1 and the secondary lines L2 istypically the same, e.g., a metal, while the material in between theprimary lines L1 and in between the secondary lines L2 may be adielectric or a semiconductor.

In the sub-segmentation the original periodicity P of the markerstructure is maintained to allow the application of the alignmentsensors from the prior art.

It is further noted that the width of the primary line L1 may or may notbe equal to the width of a secondary line L2.

The alignment system uses a first laser beam with a first linearpolarization E1 and a second laser beam with a second linearpolarization E2. The wavelength of the first laser beam differs from thewavelength of the second laser beam. For example, the first laser beamincludes a red light, and the second laser beam includes a green light.

The first linear polarization direction E1 is perpendicular to thesecond linear polarization direction E2. Further, the first linearpolarization E1 is arranged in such a way that the line segments L1 inthe marker lines allow a further transmission of the first polarizedlight beam in order to form a diffraction pattern of the markerstructure. Similarly, the second linear polarization E2 is arranged insuch a way that the line segments L2 in the intermediate marker elementsallow a further transmission of the second polarized light beam to forma diffraction pattern of the marker structure.

FIG. 9 shows an application of a stack of markers in accordance with theseventh embodiment of the present invention. A further advantage of thestructure of the seventh embodiment is the fact that at least two ofsuch markers can be stacked on top of each other, without causing anyinterference between them. By stacking marker structures in subsequentlayers, the needed estate for the marker structures in the scribelanecan be significantly reduced. In the example of FIG. 9, in such a stack,a second marker OM2 is translated over half the periodicity P withrespect to the first marker OM1, with the width of “lines” being equalto the width of “trenches”. Due to the segmentation of the “trenches”and “lines” perpendicular to each other, the polarization effectprohibits cross-talk between the upper and lower marker structure. Whenusing the first and second laser beams with their mutual perpendicularpolarization, the lower marker structure appears covered by the uppermarker: the alignment system only detects the upper marker structure.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A marker structure on a substrate for optical alignment of thesubstrate, the marker structure comprising: a plurality of firststructural elements; and a plurality of second structural elements, themarker structure capable of directing light incident thereon to a sensorfor determining alignment information, the alignment informationcomprising information relating to a position of the substrate; whereinthe first structural element has a first reflecting surface on a firstlevel and a second reflecting surface on a second level lower than thefirst level, the second structural element being substantiallynon-reflecting, a separation between the first and second reflectingsurfaces determining a phase depth condition for the detected light, anda recess provided in the second reflecting surface to modify the phasedepth condition.
 2. A marker structure according to claim 1, wherein thefirst and the second structural elements are arranged to form adiffraction grating, the first structural elements being lines of thegrating and the second structural elements being spaces of the grating.3. A marker structure according to claim 1, wherein the first structuralelements comprise a metal.
 4. A marker structure according to claim 1,wherein the second structural elements comprise a dielectric.
 5. Amarker structure according to claim 1, wherein the recess is presentunder a portion of the marker structure.
 6. A marker structure accordingto claim 1, wherein the recess is created as a partial recess, thepartial recess substantially being located under each of the secondstructural elements.
 7. A marker structure according to claim 1, whereinthe recess is created as a partial recess, the partial recesssubstantially being located under the first reflecting surface of eachof the first structural elements.
 8. A marker structure according toclaim 3, wherein the metal is copper.
 9. A marker structure on asubstrate for optical alignment of the substrate, the marker structurecomprising: a plurality of first structural elements; and a plurality ofsecond structural elements, the marker structure capable of directinglight incident thereon to a sensor for determining alignmentinformation, the alignment information comprising information relatingto a position of the substrate, wherein the first structural element hasa first reflecting surface on a first level and a second reflectingsurface on a second level lower than the first level, the secondstructural element is substantially non-reflecting, a separation betweenthe first and second reflecting surfaces determines a phase depthcondition for the detected light, and the second reflecting surfacecomprises a plurality of additional structural elements located above anopaque layer.
 10. A marker structure according to claim 9, wherein thefirst and the second structural elements are arranged to form a firstdiffraction grating, the first structural elements being lines of thegrating and the second structural elements being spaces of the grating,and the additional structural elements are arranged as lines of a seconddiffraction grating, the tone of the second diffraction grating beingsubstantially the reverse of the tone of the first diffraction grating.11. A marker structure on a substrate for optical alignment of thesubstrate, comprising: a plurality of first structural elements; and aplurality of second structural elements, wherein the first and thesecond structural elements are arranged in a repetitive order of onefirst structural element located adjacent to one second structuralelement, the marker structure having a periodicity in an orderingdirection of the repetitive order, the first structural elements eachhaving a first width in the ordering direction, the second structuralelements each having a second width in the ordering direction, the firstand second structural elements having a length direction extendingperpendicular to the ordering direction, the marker structure capable ofdiffracting light incident thereon to be received by a sensor formeasurement of a diffraction pattern, wherein the marker structurecomprises a first periodic structure and a second periodic structure,the second periodic structure is adjacent and parallel to the firstperiodic structure, the first periodic structure comprises a pluralityof the first structural elements of a first material and having a firstwidth and a plurality of the second structural elements of a secondmaterial and having a second width, the first and second structuralelements are arranged in a repetitive order with the first width beinglarger than the second width, the second periodic structure comprising aplurality of the first structural elements of the second material andhaving a third width and a plurality of the second structural elementsof the first material and having a fourth width, the first and secondstructural elements being arranged in a repetitive order, the thirdwidth is equal to the first width and the fourth width is equal to thesecond width, and the first structural elements in the second periodicstructure are located adjacent to the first structural elements in thefirst periodic structure in such a manner that the second periodicstructure is complementary to the first periodic structure.
 12. A markerstructure on a substrate for optical alignment of the substrate, themarker structure comprising: a plurality of first structural elements;and a plurality of second structural elements, the marker structurefacilitating optical alignment based upon at least one light beamdirected on the marker structure to be detected by a sensor, wherein thefirst structural elements are formed from a first material and thesecond structural elements are formed from a second material, the firstand second structural elements being arranged in a complementaryconfiguration such that in the absence of asymmetric damage to the firstand second structural elements, a first signal is detected at the sensorand in the presence of asymmetric damage to the first and secondstructural elements a second signal is detected at the sensor.
 13. Amarker structure according to claim 12, wherein the first signal is azero or minimum intensity signal, and the second signal is a largerintensity signal.
 14. A marker structure according to claim 11 or 12,wherein the first material is conductor material and the second materialis either a semiconductor or insulator material.
 15. A marker structureaccording to claim 14, wherein the first material is copper and thesecond material is a dielectric material.
 16. A marker structure on asubstrate for optical alignment of the substrate, the marker structurecomprising: a plurality of first structural elements; and a plurality ofsecond structural elements, the marker structure facilitating opticalalignment based upon at least one light beam directed on the markerstructure to be detected by a sensor, wherein the marker structure ispresent in a metallization layer of the substrate, at least one of thefirst structural elements includes a first surface area portion having afirst surface state and at least one of the second structural elementsincludes a second surface area portion having a second surface state,the first surface area portion is related to a first buried markerelement, and the second surface area portion is related to a secondburied marker element, the first and the second surface states arerelated to variations in morphology of the metallization layer beinginduced by the first buried marker element and the second buried markerelement, respectively.
 17. A marker structure according to claim 16,wherein the first and the second structural elements are arranged toform a diffraction grating.
 18. A marker structure according to claim16, wherein the metallization layer comprises a metal layer beingdeposited by a hot metal deposition process in a metallizationprocessing sequence.
 19. A marker structure according to claim 16,wherein the metallization layer comprises an aluminium layer.
 20. Amarker structure according to claim 18, wherein the metallizationprocessing sequence further comprises at least one of a deposition of aTi adhesion layer, or a deposition of a Ti/TiN capping layer, or adeposition of a passivation layer, or any combination of the foregoing.21. A marker structure on a substrate for optical alignment of thesubstrate, the marker structure comprising: a plurality of firststructural elements; and a plurality of second structural elements, themarker structure capable of directing light incident thereon to asensor, wherein the first structural elements comprise a plurality ofprimary lines comprising a first material and a plurality of firstinterposed lines comprising a second different material.
 22. A markerstructure according to claim 21, wherein the first and the secondstructural elements are arranged to form a diffraction grating.
 23. Amarker structure according to claim 21, wherein the first material has afirst resistance to chemical mechanical polishing, the second materialhas a second resistance to chemical mechanical polishing, and the firstresistance is different from the second resistance.
 24. A markerstructure according to claim 22, wherein the plurality of firstinterposed lines form a periodic structure.
 25. A marker structureaccording to claim 24, wherein the periodic structure extends in adirection substantially perpendicular to a periodic direction of thediffraction grating.
 26. A marker structure according to claim 24,wherein the periodic structure extends in a direction substantiallyparallel to a periodic direction of the diffraction grating.
 27. Amarker structure according to claim 25, wherein the second structuralelement comprises a plurality of secondary lines and a plurality ofsecond interposed lines, the plurality of second interposed linesforming a further periodic structure in a direction substantiallyperpendicular to the direction of the periodic structure formed by theplurality of first interposed lines.
 28. A marker structure according toclaim 21, wherein the primary lines and the first interposed lines havea dimension comparable to a critical feature size of a product devicebeing created on the substrate.
 29. A marker structure according toclaim 27, wherein the secondary lines and the second interposed lineshave a dimension comparable to a critical feature size of a productdevice being created on the substrate.
 30. A substrate formicroelectronic devices comprising at least one marker structureaccording to claim 1, 9, 11, 12, 16 or
 21. 31. A lithographic projectionapparatus, comprising: a radiation system configured to provide aprojection beam of radiation; a support configured to support apatterning device, the patterning device configured to pattern theprojection beam according to a desired pattern; a substrate tableconfigured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate; asubstrate alignment system configured to detect a position of thesubstrate relative to a position of the patterning device; the substratecomprising at least one marker structure according to claim 1, 9, 11,12, 16 or
 21. 32. A method of alignment of a substrate in a lithographicprojection apparatus, the method comprising: providing at least onelight beam directed on a marker structure, the marker structureincluding a plurality of first structural elements and a plurality ofsecond structural elements, the marker structure capable of directinglight incident thereon to a sensor for determining alignmentinformation, wherein the first structural element has a first reflectingsurface on a first level and a second reflecting surface on a secondlevel lower than the first level, the second structural element beingsubstantially non-reflecting, a separation between the first and secondreflecting surfaces determining a phase depth condition for the detectedlight, and a recess provided in the second reflecting surface to modifythe phase depth condition; determining alignment information from thelight received from the marker structure at the sensor; and aligning thesubstrate according to the determined alignment information.
 33. Amethod of alignment of a substrate in a lithographic projectionapparatus, the method comprising: providing at least one light beamdirected on a marker structure, the marker structure including aplurality of first structural elements and a plurality of secondstructural elements, the marker structure capable of directing lightincident thereon to a sensor for determining alignment information,wherein the first structural element has a first reflecting surface on afirst level and a second reflecting surface on a second level lower thanthe first level, the second structural element being substantiallynon-reflecting, a separation between the first and second reflectingsurfaces determining a phase depth condition for detected light, and thesecond reflecting surface comprises a plurality of additional structuralelements located above an opaque layer; determining alignmentinformation from the light received from the marker structure at thesensor; and aligning the substrate according to the determined alignmentinformation.
 34. A method of alignment of a substrate in a lithographicprojection apparatus, the method comprising: providing at least onelight beam directed on a marker structure, the marker structureincluding a plurality of first structural elements and a plurality ofsecond structural elements, wherein the first and the second structuralelements are arranged in a repetitive order of one first structuralelement located adjacent to one second structural element, the markerstructure having a periodicity in an ordering direction of therepetitive order, the first structural elements each having a firstwidth in the ordering direction, the second structural elements eachhaving a second width in the ordering direction, the first and secondstructural elements having a length direction extending perpendicular tothe ordering direction, the marker structure capable of diffractinglight incident thereon to be received by a sensor for measurement of adiffraction pattern, wherein the marker structure comprises a firstperiodic structure and a second periodic structure, the second periodicstructure is adjacent and parallel to the first periodic structure, thefirst periodic structure comprises a plurality of the first structuralelements of a first material and having a first width and a plurality ofthe second structural elements of a second material and having a secondwidth, the first and second structural elements are arranged in arepetitive order with the first width being larger than the secondwidth, the second periodic structure comprising a plurality of the firststructural elements of the second material and having a third width anda plurality of the second structural elements of the first material andhaving a fourth width, the first and second structural elements arearranged in a repetitive order, the third width is equal to the firstwidth and the fourth width is equal to the second width, and the firststructural elements in the second periodic structure are locatedadjacent to the first structural elements in the first periodicstructure in such a manner that the second periodic structure iscomplementary to the first periodic structure; determining alignmentinformation from the light received from the marker structure at asensor; and aligning the substrate according to the determined alignmentinformation.
 35. A method of alignment of a substrate in a lithographicprojection apparatus, the method comprising: providing at least onelight beam directed on a marker structure, the marker structureincluding a plurality of first structural elements and a plurality ofsecond structural elements, the marker structure facilitating opticalalignment based upon at least one light beam directed on the markerstructure to be detected by a sensor, wherein the first structuralelements are formed from a first material and the second structuralelements are formed from a second material, the first and secondstructural elements being arranged in a complementary configuration suchthat in the absence of asymmetric damage to the first and secondstructural elements, a first signal is detected at the sensor and in thepresence of asymmetric damage to the first and second structuralelements a second signal is detected at the sensor; determiningalignment information from the light received from the marker structureat the sensor; and aligning the substrate according to the determinedalignment information.
 36. A method of alignment of a substrate in alithographic projection apparatus, the method comprising: providing atleast one light beam directed on a marker structure, the markerstructure including a plurality of first structural elements and aplurality of second structural elements, the marker structurefacilitating optical alignment based upon at least one light beamdirected on the marker structure to be detected by a sensor, wherein themarker structure is present in a metallization layer, the firststructural element includes a first surface area portion having a firstsurface state and the second structural element includes a secondsurface area portion having a second surface state, the first surfacearea portion is related to a first buried marker element, and the secondsurface area portion is related to a second buried marker element, thefirst and the second surface states are related to variations inmorphology of the metallization layer being induced by the first buriedmarker element and the second buried marker element, respectively;determining alignment information from the light received from themarker structure at the sensor; and aligning the substrate according tothe determined alignment information.
 37. A method of alignment of asubstrate in a lithographic projection apparatus, the method comprising:providing at least one light beam directed on a marker structure, themarker structure comprising a plurality of structural elements andcapable of directing light incident thereon to a sensor, wherein thestructural elements comprise a plurality of primary lines comprising afirst material and a plurality of first interposed lines comprising asecond different material; detecting alignment information from thelight received from the marker structure at the sensor; and aligning thesubstrate according to the detected alignment information.